fix: Port "non-clipping subtree" logic from WinUI

src/Uno.UI/UI/Xaml/Controls/ItemsControl/ItemsPresenter.cs

@@ -358,6 +358,37 @@ private bool WantsScrollViewerToObscureAvailableSizeBasedOnScrollBarVisibility(O
 			return (spPanel as FrameworkElement)?.WantsScrollViewerToObscureAvailableSizeBasedOnScrollBarVisibility(orientation) ?? true;
 		}
 
+		// Itemspresenter is the lynchpin in deciding whether we want to be in a non clipping subtree.
+		// Basically, when he decided to not want infinity, even though we were in a scrolling configuration,
+		// we want the measures underneath it to not constrain the desiredsize to the availablesize. That will allow
+		// wrapping uielements (like textblock) to size correctly, and still allow containers that want to be bigger
+		// to look good.
+		// Itemspresenter will set the value on himself (used during layout) and on the panel. Listview will set it on the
+		// items (so that individual measure on them will work).
+		internal bool EvaluateAndSetNonClippingBehavior(bool isNotBeingPassedInfinity)
+		{
+			bool result = false;
+			if (Panel is ItemsStackPanel panel)
+			{
+				// we only want the behavior in an isp at the moment
+
+				// first set it to ourselves
+				IsNonClippingSubtree = isNotBeingPassedInfinity;
+
+				// in the case of a modernpanel, lets have this itemspresenter not clip to the available size
+				// in overconstrained scenarios
+				// we're just passing through the setting to the panel here in an effort to make itemspresenter behave like a
+				// 'dumb' passthrough. The real logic was performed when measuring the scrollcontentpresenter
+
+				// we pass it along - the reason we want these individual elements also to have the correct value is because
+				// they could potentially be measured independently
+				panel.IsNonClippingSubtree = isNotBeingPassedInfinity;
+
+				result = isNotBeingPassedInfinity;
+			}
+
+			return result;
+		}
 
 		protected override Size ArrangeOverride(Size finalSize)
 		{

src/Uno.UI/UI/Xaml/Controls/ScrollContentPresenter/ScrollContentPresenter.cs

@@ -186,24 +186,32 @@ protected override Size MeasureOverride(Size availableSize)
 					}
 				}
 
+				// when set to true, this means that we wanted to set to infinity but were blocked in doing it.
+				bool childPreventsInfiniteAvailableWidth = false;
+				bool childPreventsInfiniteAvailableHeight = false;
 
 				if (CanVerticallyScroll)
 				{
-					var childPreventsInfiniteAvailableHeight = !child.WantsScrollViewerToObscureAvailableSizeBasedOnScrollBarVisibility(Orientation.Vertical);
+					childPreventsInfiniteAvailableHeight = !child.WantsScrollViewerToObscureAvailableSizeBasedOnScrollBarVisibility(Orientation.Vertical);
 					if (!sizesContentToTemplatedParent && !childPreventsInfiniteAvailableHeight)
 					{
 						slotSize.Height = double.PositiveInfinity;
 					}
 				}
 				if (CanHorizontallyScroll)
 				{
-					var childPreventsInfiniteAvailableWidth = !child.WantsScrollViewerToObscureAvailableSizeBasedOnScrollBarVisibility(Orientation.Horizontal);
+					childPreventsInfiniteAvailableWidth = !child.WantsScrollViewerToObscureAvailableSizeBasedOnScrollBarVisibility(Orientation.Horizontal);
 					if (!sizesContentToTemplatedParent && !childPreventsInfiniteAvailableWidth)
 					{
 						slotSize.Width = double.PositiveInfinity;
 					}
 				}
 
+				if (child is ItemsPresenter itemsPresenter)
+				{
+					itemsPresenter.EvaluateAndSetNonClippingBehavior(childPreventsInfiniteAvailableWidth || childPreventsInfiniteAvailableHeight);
+				}
+
 				child.Measure(slotSize);
 
 				var desired = child.DesiredSize;

src/Uno.UI/UI/Xaml/FrameworkElement.Layout.crossruntime.cs

@@ -309,9 +309,7 @@ private void InnerMeasureCore(Size availableSize)
 
 				// only clip and constrain if the tree wants that.
 				// currently only listviewitems do not want clipping
-				// UNO TODO
-
-				//if (!pLayoutManager->GetIsInNonClippingTree())
+				if (!IsInNonClippingTree)
 				{
 					// In overconstrained scenario, parent wins and measured size of the child,
 					// including any sizes set or computed, can not be larger then

src/Uno.UI/UI/Xaml/UIElement.Layout.crossruntime.cs

@@ -20,6 +20,9 @@ public partial class UIElement : DependencyObject
 		/// </summary>
 		internal bool ShouldInterceptInvalidate { get; set; }
 
+		// In WinUI, this is in LayoutManager. For Uno, we make it static in FE for now.
+		private protected static bool IsInNonClippingTree { get; set; }
+
 		public void InvalidateMeasure()
 		{
 			if (ShouldInterceptInvalidate || IsMeasureDirty || IsLayoutFlagSet(LayoutFlag.MeasuringSelf))
@@ -213,6 +216,12 @@ private void DoMeasure(Size availableSize)
 
 			var remainingTries = MaxLayoutIterations;
 
+			// remember whether we need to set this value back
+			var wasInNonClippingTree = IsInNonClippingTree;
+			// once entering a tree that is non-clipping, we don't get out of it
+			// remember this for down-stream
+			IsInNonClippingTree = IsNonClippingSubtree || wasInNonClippingTree;
+
 			while (--remainingTries > 0)
 			{
 				if (isDirty)
@@ -306,6 +315,8 @@ private void DoMeasure(Size availableSize)
 
 				break;
 			}
+
+			IsInNonClippingTree = wasInNonClippingTree;
 		}
 
 		internal virtual void MeasureCore(Size availableSize)

src/Uno.UI/UI/Xaml/UIElement.mux.cs

@@ -1933,5 +1933,7 @@ internal virtual void LeaveImpl(LeaveParams @params)
 
 		internal virtual bool WantsScrollViewerToObscureAvailableSizeBasedOnScrollBarVisibility(Orientation horizontal)
 			=> true;
+
+		internal bool IsNonClippingSubtree { get; set; }
 	}
 }